1. Field
The present disclosure relates to a reception technology, and more specifically, to an FM receiver and an FM receiving method for receiving FM signals.
2. Description of the Related Art
A direct conversion type Frequency Modulation (FM) receiver converts an RF signal into a baseband signal by quadrature detection, and amplifies the baseband signal by an amplifier. When unnecessary Direct Current (DC) components are output by the amplifier, such DC components cause a DC offset, deteriorating the signal receiving performance of the receiver. In order to avoid such a performance deterioration, polar coordinate conversion is performed on an I-phase baseband signal and a Q-phase baseband signal to obtain an amplitude signal and a phase signal, the phase signal is mapped into four phase domains, and an average value of the amplitude signals for each phase domain is derived. Accordingly, a displacement of the baseband signal from an origin is derived. This displacement is taken as a correction value for the DC offset, and the I-phase baseband signal and the Q-phase baseband signal are corrected accordingly (see, for example, patent document 1).
[patent document 1] JP2011-29717
When a DC offset is detected by utilizing characteristics of a baseband Lissajous waveform in a constant envelope modulation scheme, and if a received signal is in an unmodulated state and the frequency thereof is identical with that of a local oscillation signal, the I-phase baseband signal and the Q-phase baseband signal take respective constant values. In addition, phase signals obtained by performing polar coordination conversion on those signals also take respective constant values, and are always mapped into a single phase domain, and thus it is difficult to distinguish the received signal and the DC offset from each other. Hence, a correction is performed in such a way that a value obtained by synthesizing the received signal with the DC offset becomes “0”, and thus the corrected I-phase baseband signal and the corrected Q-phase baseband signal become “0”. Hence, obtainment of a normal detection signal is difficult. In addition, when a modulation index of the received signal is small, the phase change in the I-phase baseband signal and in the Q-phase baseband signal becomes little. Accordingly, the similar phenomenon occurs.